Process for preparing alkali metal salts of 3-hydroxypropionic acid

ABSTRACT

PCT No. PCT/EP92/00477 Sec. 371 Date Sep. 13, 1993 Sec. 102(e) Date Sep. 13, 1993 PCT Filed Mar. 4, 1992 PCT Pub. No. WO92/16489 PCT Pub. Date Oct. 1, 1992.By reacting 1,3-propanediol with oxygen or an oxygen-containing gas in an aqueous alkaline solution in the presence of a palladium containing catalyst substrate, alkaline salts of 3-hydroxypropionic acid are produced in good yield when the catalyst is used in an amount that corresponds to 0.1 to 3.0% by weight of palladium, based on the 1,3-propanediol.

FIELD OF THE INVENTION

This invention relates to a process for the production of alkali metalsalts of 3-hydroxypropionic acid by oxidation of propane-1,3-diol inaqueous alkaline solution in the presence of a palladium catalyst.

STATEMENT OF RELATED ART

3-Hydroxypropionic acid and its salts are valuable building blocks inorganic synthesis. 3-Hydroxypropionic acid is normally prepared byaddition of water onto acrylic acid or by reaction of ethylenechlorohydrin with sodium cyanide and subsequent hydrolysis of theβ-propiolactone formed [Ullmann's Encyclopoedia of Industrial Chemistry,5th Edition, Vol. A-13, pages 507-517]. Both processes involve thehandling of toxic substances. Accordingly, a search was made for aprocess by which toxicologically safe propane-1,3-diol, which canreadily be obtained in high yields from glycerol by fermentation, couldbe converted by oxidation into 3-hydroxypropionic acid or alkali metalsalts thereof.

The oxidation of propane-1,3-diol, hereinafter referred to in brief asdiol, in presence of a noble metal catalyst is known from theliterature. Thus, published Japanese patent application JP 56/5433(Sanyo) claims a process for the production of malonic acid by reactionof propane-1,3-diol with oxygen or an oxygen-containing gas. The processis preferably carried out in the presence of a platinum group catalyst.According to the Sanyo application, malonic acid can be obtained in highyields using 3.3% by weight palladium, based on the diol.

DESCRIPTION OF THE INVENTION Object of the Invention

The problem addressed by the present invention was to provide a processby which propane-1,3-diol could be oxidized in high yields to3-hydroxypropionic acid.

Summary of the Invention

According to the invention, this problem has been solved by a processfor the production of an alkali metal salt of 3-hydroxypropionic acid byreaction of propane-1,3-diol with oxygen or an oxygen-containing gas inaqueous alkaline solution in the presence of a palladium-containingsupported catalyst, characterized in that the catalyst is used in aquantity corresponding to 0.1 to 3.0% by weight of palladium, based onpropane-1,3-diol.

Description of the Preferred Embodiments

Palladium on a solid support, for example active carbon or aluminumoxide, is used as the catalyst, the quantity of palladium in thecatalyst being from 0.5 to 10% by weight and preferably from 2 to 5% byweight, based on the support.

The practicability of the process according to the invention isattributable to the fact that the predominant formation of3-hydroxypropionic acid (selective oxidation of propane-1,3-diol) takespriority over the competitive formation of malonic acid (completeoxidation of propane-1,3-diol) where the low catalyst concentrationaccording to the invention, based on the diol, is used.

In one preferred embodiment of the invention, the quantity of palladiumpresent in the catalyst, based on propane-1,3-diol, is between 0.1 and1.0% by weight.

It has been found that high yields of 3-hydroxypropionic acid areobtained even when, over and above the palladium present, the catalystadditionally contains platinum and/or bismuth. The total quantity ofplatinum and/or bismuth should be at most twice the quantity by weightof palladium. Particularly good results have been obtained with acatalyst containing 4% by weight of palladium, 1% by weight of platinumand 5% by weight of bismuth on powdered active carbon.

The catalyst is normally activated before use. This is readily done bydispersing the catalyst in water and subsequently contacting it withinert gases, for example hydrogen and/or nitrogen, to displace adheringoxygen.

The catalyst may be repeatedly reused in the process according to theinvention without any losses of yield having to be accepted. On thecontrary, it has even been found that the catalyst only develops itsfull activity after it has been used at least once to three times.

The concentration of the diol in the reaction mixture is not subject toany particular limitations, although a concentration of 5 to 20% byweight is preferred. It is of particular advantage in this regard toadjust the concentration of the diol in the reaction mixture to a valueof 6 to 12% by weight and, more particularly, to a value of 8 to 10% byweight. In this case, the quantity of palladium present in the catalystcan be further reduced to 0.1 to 0.3% by weight of the same high yield.

According to the invention, the oxidation of propane-1,3-diol is carriedout in alkaline medium. In this way, the 3-hydroxypropionic acid formedis neutralized and thus protected against partial yield-reducingdegradation by decarboxylation. The pH value of the aqueous alkalinereaction mixtures should be in the range from 8 to 13 and is preferablyin the range from 9 to 12. Particularly good results are obtained at pHvalues of 10 to 11.

Oxidation of the diol is carried out at a constant pH value of thereaction mixture throughout the reaction. The constant pH value may becontrolled, for example, by coupling a pH meter, which continuouslymonitors the pH value of the reaction mixture, to a metering unitcontaining the corresponding alkali metal hydroxide. The alkali metalhydroxides used are preferably sodium hydroxide and potassium hydroxide,more particularly in the form of aqueous solutions. The concentration ofthe aqueous alkali metal hydroxide used is not subject to any particularlimitation, although it is clear that the use of highly dilute solutionsis uneconomical in regard to optimal utilization of the reactor. Forpractical reasons, therefore, 20 to 50% by weight of aqueous alkalimetal hydroxides will be used. In the case of NaOH, it has proved to bepractical to use a 30% by weight, i.e. 10-normal, solution.

In the process according to the invention, oxidation of the diol ispreferably carried out at temperatures of 40° to 55° C. Highertemperatures do not produce any significant increases in yield andactually involve the danger of dehydration of the 3-hydroxypropionicacid to acrylic acid.

In the process according to the invention, the reaction mixture iscontacted with oxygen or an oxygen-containing gas, for example air. Thismay readily be done, for example, by injecting air into the reactionmixture with stirring. It has been found that a flow rate of 30 normalliters of air per hour, based on 700 to 1000 ml reaction mixture, isparticularly suitable. At lower flow rates, for example 10 normal litersair per hour, the reaction time is significantly extended and thereaction mixture can undergo unwanted yellowing; at higher flow rates,the removal of catalyst from the reactor is too high.

The reaction may be carried out under pressures of 1.0 to 1.5 bar, butis preferably carried out under a slight excess pressure of the order of1.01 to 1.06 bar.

In kinetic studies, it was found that the consumption of alkali metalhydroxide in the process according to the invention is dependent ontime. The consumption of alkali metal hydroxide initially increaseslinearly as a function of time, reaching a plateau value at the end ofthe reaction. On the basis of this observation, therefore, the end pointof the reaction may readily be determined from the fact that no morealkali metal hydroxide is needed to keep the pH value constant. At thisstage, the reaction is terminated and the reaction mixture is worked upin the usual way beginning with removal of the catalyst by filtration.

The alkali metal salt of 3-hydroxypropionic acid obtained may be usedeither directly or after further concentration in the form of an aqueoussolution. If desired, the alkali metal salt may be converted byacidification, for example by means of an acidic ion exchanger, into thefree 3-hydroxypropionic acid, which may optionally be purified bydistillation.

The following Examples are intended to illustrate the invention.

Examples

1. Reagents

The following catalysts were used:

a) 5% by weight Pd on active carbon with a water content of 52.5% byweight, commercially available under the name of Escat 10 (Engelhard).

b) 4% by weight Pd, 1% by weight Pt and 5% by weight Bi on active carbonwith a water content of 59.3% by weight, commercially available underthe name of Cef 196 raw (Degussa AG).

2. Test apparatus

The oxidations were carried out in a 2 liter pressure autoclave equippedwith a turbine stirrer [1400 revolutions per minute]. Air and 30% byweight sodium hydroxide were introduced into the reaction mixture frombelow. NaOH was continuously introduced at such a rate that the pH valueof the mixture remained constant. The pH was controlled through ametering dispenser (Dulcometer, manufacturer: Prominent) equipped with aresistance thermometer (Pt-100). The waste gas passed through a cooler(deposition of condensate), a buffer vessel, a water-filled washingbottle, a drying tower, a throughflow meter and an oxygen analyzer(Servomex 570, manufacturer: Buhler). Due to variations in air pressure,the instrument was recalibrated before each test. The consumption ofoxygen was continuously recorded as a function of time by a connectedrecorder. The air throughput was adjusted by a precision control valveto a value of 30 normal liters per hour. The pressure inside the reactorwas 1.06 bar.

3. Test descriptions

The quantities of catalyst shown in all the Examples and ComparisonExamples are based on dry matter.

Example 1 (E1)

2.13 g of catalyst (Escat 10) were dispersed in 300 ml of water andactivated overnight under hydrogen, a quantity of about 350 ml beingtaken up. The prepared catalyst was transferred to the autoclavetogether with 77.6 g (1 mole) of propane-1,3-diol and another 500 mlwater. The autoclave was closed and the reaction mixture was heatedunder nitrogen to 50° C. The reaction was initiated by the simultaneousintroduction of sodium hydroxide and air. NaOH was continuouslyintroduced at such a rate that the pH value remained constant at 11. Theair throughput was 30 normal liters per hour. As soon a constant valuehad been reached for the total amount of sodium hydroxide solutionintroduced as well as for the therewith connected total consumption ofoxygen, the reaction was terminated, the reaction mixture was drainedoff by blowing out the NaOH feed line, and the yield was determined byweighing. After cooling of the mixture, the catalyst was filtered offthrough a suction filter and the samples were analyzed by HPLC (ShodexIonpak C-811: cation exchange phase, 0.1% by weight aqueous phosphoricacid as eluent, Ri detection). The yields of 3-hydroxypropionic acid [in% of the theoretical] are shown together with other data in line 1 ofTable 1.

                                      TABLE 1                                     __________________________________________________________________________        Propane-1,3-diol                                                                       Water                                                                             Cat..sup.a)                                                                       Pd.sup.b)                                                                        Temp.                                                                             Time.sup.c)                                                                       Yield.sup.d)                                  Ex. [g] [mmoles]                                                                           [g] [g] [%]                                                                              [°C.]                                                                      [mins.]                                                                           [%]                                           __________________________________________________________________________    E1  77.6                                                                              1000 800 2.13                                                                              0.14                                                                             50  1075.sup.e)                                                                       70.1                                          E2  77.6                                                                              1000 800 3.80                                                                              0.24                                                                             50   490                                                                              74.7                                          E3  77.6                                                                              1000 800 5.32                                                                              0.34                                                                             50   415                                                                              68.4                                          E4  77.6                                                                              1000 800 10.64                                                                             0.69                                                                             50   330                                                                              51.5                                          C1  77.6                                                                              1000 800 21.28                                                                             1.37                                                                             50   580                                                                              23.5                                          E5  23.3                                                                               300 700 13.10                                                                             2.81                                                                             40   190                                                                              65.6                                          C2  23.3                                                                               300 700 13.10                                                                             2.81                                                                             60   220                                                                              29.0                                          __________________________________________________________________________     .sup.a) Escat 10; the quantities of catalyst shown are dry weights            .sup.b) % by weight palladium, based on propane1,3-diol                       .sup.c) Reaction time in minutes                                              .sup.d) Yield of 3hydroxypropionic acid in % of the theoretical               .sup.e) Air throughput: 10 normal liters per hour                        

Examples 2 to 4 (E2 to E4)

Example 1 was repeated with different quantities of catalyst. Theresults are set out in Table 1. It can be seen that optimal yields of3-hydroxypropionic acid are obtained in particular at low catalystconcentrations.

Comparison Example 1 (C1)

Example 1 was repeated with a distinctly larger quantity of catalyst.Particulars are set out in Table 1. It can be seen that the increase inthe quantity of catalyst is accompanied by a drastic reduction in theyield of 3-hydroxypropionic acid.

Example 5 (E5)

13.1 kg of catalyst (Escat 10) were dispersed in 300 ml of water andactivated overnight under hydrogen, a quantity of about 900 ml beingtaken up. The prepared catalyst was transferred to the autoclavetogether with 23.3 g (300 mmoles) of propane-1,3-diol and another 400 mlof water. The autoclave was closed and the reaction mixture was heatedunder nitrogen to 40° C. The remaining procedure was as in Example 1.The yield of 3-hydroxypropionic acid was 65.6%, cf. Table 1.

Comparison Example 2 (C2)

Example 5 was repeated at 60° C. The yield of 3-hydroxypropionic acidwas 29%, cf. Table 1.

Example 6 (E6)

13.1 kg of catalyst (Cef 196 raw) were dispersed in 300 ml of water andactivated overnight under hydrogen, a quantity of about 900 ml beingtaken up. The prepared catalyst was transferred to the autoclavetogether with 31.1 g (400 mmoles) of propane-1,3-diol and another 631 mlof water. The autoclave was closed and the reaction mixture was heatedunder nitrogen to 50° C. The remaining procedure was as in Example 1.The yield of 3-hydroxypropionic acid was 81.8%, cf. Table 2.

Example 7 to 10 (E7 to E10)

Example 6 was repeated with different concentrations of propane-1,3-diolin the reaction mixture and hence indirectly with different ratios ofcatalyst to diol. Particulars and also the yields of 3-hydroxypropionicacid are set out in Table 2.

                                      TABLE 2                                     __________________________________________________________________________       Propane-1,3-diol                                                                        Water                                                                            Cat..sup.a)                                                                       Pd.sup.b)                                                                        Met.sup.c)                                                                       Temp.                                                                             Time.sup.d)                                                                       Yield.sup.e)                                Ex.                                                                              [g] [mmoles]                                                                            [g]                                                                              [g] [%]                                                                              [%]                                                                              [°C.]                                                                      [mins.]                                                                           [%]                                         __________________________________________________________________________    E6 31.1                                                                              400  931 17.46                                                                             2.2                                                                              3.4                                                                              50  140 81.8                                        E7 23.3                                                                              300  700 13.10                                                                             2.3                                                                              3.3                                                                              50  140 73.8                                        E8 46.6                                                                              600  700 13.10                                                                             1.1                                                                              1.7                                                                              50  240 77.2                                        E9 69.8                                                                              900  700 13.10                                                                             0.8                                                                              1.1                                                                              50  300 78.2                                        E10                                                                              93.1                                                                              1200 700 13.10                                                                             0.6                                                                              0.8                                                                              50  570 70.5                                        __________________________________________________________________________     .sup.a) Cef 196 raw; the quantities of catalyst shown are dry weights         .sup.b) % by weight palladium, based on propane1,3-diol                       .sup.c) % by weight platinum + bismuth (based on propane1,3-diol              .sup.d) Reaction time in minutes                                              .sup.e) Yield of 3hydroxypropionic acid in % of the theoretical          

The invention claimed is:
 1. A process for the production of an alkalimetal salt of 3-hydroxypropionic acid by reaction of propane-1,3-diolwith oxygen or an oxygen-containing gas in aqueous alkaline solution inthe presence of a palladium-containing supported catalyst, wherein:(a)the catalyst is used in a quantity corresponding to 0.1 to 3.0% byweight of palladium, based on propane-1,3-diol and (b) the reaction iscarried out at a temperature of 40° to 55° C.
 2. A process as claimed inclaim 1, wherein the catalyst is used in a quantity corresponding to 0.1to 1.0% by weight of palladium, based on propane-1,3-diol.
 3. A processas claimed in claim 2, wherein propane-1,3-diol is used in a quantitycorresponding to 6 to 12% by weight, based on the reaction mixture.
 4. Aprocess as claimed in claim 3, wherein the catalyst is used in aquantity corresponding to 0.1 to 0.3% by weight of palladium, based onpropane-1,3-diol.
 5. A process as claimed in claim 1, wherein a catalystadditionally containing platinum, bismuth, or both platinum and bismuthis used.
 6. A process as claimed in claim 5, wherein a catalystcontaining 4% by weight of palladium, 1% by weight of platinum and 5% byweight of bismuth is used.